A gas insulated switchgear is configured to include a plurality of containers for housing a disconnector, a circuit breaker and the like therein and coupled to each other to form a structured body that is connected to a transmission line. Generally, a conductor member supplied with high electric power is arranged inside the above-described container. In order to suppress the conductor member supplied with high electric power from being unintentionally electrically conductive to other regions (particularly, a container) inside the container, the container is filled with insulating gas (insulating medium) or a conductor member is supported inside the container by an insulating support member (insulating spacer) made of an insulating member. By such a structure, the conductor member of the gas insulated switchgear is protected from the influence of the external world. Accordingly, the gas insulated switchgear is improved in reliability and safety, and also decreased in load to the environment.
Examples of the above-described insulating gas generally used for the gas insulated electrical apparatus may be sulfur hexafluoride gas (SF6 gas), dry air, nitrogen, carbon dioxide gas, carbon fluoride (CF4), iodomethane (CHI3), hexafluoroethane (C2F6), octafluoropropane (C3F8), or the like, or mixed gas obtained by combining these gases. Particularly, sulfur hexafluoride gas has dielectric strength about 3 times as much as air. Accordingly, by using sulfur hexafluoride gas as insulating gas in the gas insulated electrical apparatus, the distance between a high voltage portion and a ground electrode can be decreased while maintaining the insulation performance and the interrupting performance. Consequently, the gas insulated electrical apparatus can be reduced in size.
Furthermore, in the gas insulated electrical apparatus, the above-described insulating gas is generally pressurized to atmospheric pressure or more during its use in order to improve the insulation performance and the interrupting performance. Thus, in order to tightly encapsulate the gas and to maintain the equal insulation distance, a high voltage conductor is employed, which is formed in a cylindrical shape and located coaxial with a tank that is similarly formed in a cylindrical shape and that serves as the above-described airtight container.
In the gas insulated electrical apparatus, when the area of the electrode serving as a high electric field portion such as a high voltage conductor is increased, the electrode area effect of reducing a breakdown electric field becomes remarkable. This phenomenon appears particularly remarkably when an electric field portion exhibiting a high electric field of a certain level or higher exists over an electrode area of several 1000 mm2 to several 100000 mm2 or more as in the case of a gas insulated electrical apparatus. In order to raise the withstand voltage while excluding the influence of the electrode area effect, it is a common practice to form a coating film (insulating coating film) having electrical insulation properties against the electrode.
In general, an electrode has a surface provided with projections and depressions of μm order. An insulating coating film is formed on this surface of the electrode, so that field-emitting electrons generated from the top portion of each projected portion can be suppressed while the electric field concentration on the top portion of each projected portion can be alleviated. Consequently, occurrence of electric discharge in the vicinity of the electrode can be suppressed, so that the withstand voltage of the gas insulated electrical apparatus is improved.
Japanese Patent Laying-Open No. 62-141909 (PTD 1) discloses that, by taking advantage of the feature of a high voltage conductor generally made of aluminum, such a high voltage conductor is anodized to form a dielectric film (alumite film), which is used as an insulating coating film.